A substantial and persistent problem in the cable industry is the ingress of moisture and water into the cable. Such ingress often results from changes in ambient conditions which cause differences in vapor pressure between the inside and the outside of a cable jacket. Consequently, moisture tends to diffuse in a unidirectional manner from the outside of the cable to the inside of the cable, resulting in an undesirably high moisture level inside the cable. High levels of condensed moisture inside a cable sheath system may have a detrimental effect on the transmission characteristics of a metallic conductor cable.
Furthermore, water may enter the cable as a result of damage to the cable which perforates the external water barrier elements. For example, animal attacks and mechanical impacts often create openings in the sheath system of the cable, which allows water to enter the cable. Once inside the cable sheath system, the water tends to move longitudinally along the cable into splice closures, if not controlled.
With respect to optical fiber cable, while the presence of water itself within the cable is not necessarily detrimental to its performance, passage of the water along the cable interior to splice junctions or terminals or associated equipment inside closures, for example, may cause problems especially in freezing environments.
Cables for transmitting communications signals must meet industry standards with respect to water blocking provisions. For example, one industry standard requires that there be no transmission of water under a pressure head of one meter in one hour through a one meter length of cable. Thus, there are numerous technical and commercial reasons to develop and implement effective water blocking compositions to be used in the cable industry.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and along the core. For example, a metallic shield used to protect a metallic conductor cable against lightning and rodent attacks is provided with a sealed longitudinal seam. Forming of a shield about a cable core requires the use of relatively low manufacturing line speeds. Also, the use of a metallic shield is destructive of the otherwise all-dielectric property of an optical fiber cable.
Because lightning strikes may cause holes in a metallic shield, it is not uncommon to include additional provisions for preventing the ingress of water into the core. Water blocking materials have been used to fill cable cores and to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. Although the use of a filling material, in the form of a grease, causes housekeeping problems, inhibits line speeds because of the need to fill carefully interstices of the cable core and presents problems for field personnel during splicing operations, for example, it continues to be used to prevent entry of the water into the core.
Another approach involves the use of a water swellable tape within the cable sheath system. The tape is used to prevent the travel of water through the sheath system as well as blocking its travel longitudinally along the cable to closures and termination points, for example. Such a tape generally is laminated, including a water swellable powder which is trapped between two non-woven tissues. Although such a tape provides suitable water protection for the cable it is relatively expensive and thick. If the tape is too thick, the diameter of the cable is increased, thereby causing problems in terminating the cable with standard size hardware. Another drawback to this tape is the problems associated with its delamination.
U.S. Pat. No. 4,867,526, discloses a cable having water blocking provisions wherein an elongated substrate member which comprises an impregnated non-metallic, non-woven, weblike material in the form of a tape is interposed between a core and a jacket. The impregnating material may comprise a film of a water swelling or so-called superabsorbent material. The water blocking substrate member is impregnated with a material which is held in the non-woven, web-like member in suspension without its being reacted. When exposed to water, the impregnating material reacts to swell and cause the tape to prevent the passage of water through the sheath system toward the core and its migration in a direction longitudinally along the cable.
A drawback to this composition is that the impregnating material may become loose and dislodged from the substrate. The water absorbing material often is lost during its application to the cable, thus reducing its ability to block water uniformly along its entire length. Yet another disadvantage of this water absorbing composition is its high cost of production due to the relatively high level of moisture that must be driven off.
Another approach to inhibiting the ingress of water involves including in a cable a water blocking yarn interposed between a core tube and an outer surface of a jacket of the cable's sheath system. The yarn extends linearly along the cable or may be wrapped helically about a portion of the sheath system. The yarn may be one which is composed of a superabsorbent fiber material which upon contact with water swells and inhibits the movement of water within the cable. Such yarns generally lack the tensile strength and thickness required for inclusion into cable. Additionally, absorbancy tends to be minimal.
U.S. Pat. No. 4,738,867 discloses a process for preparing a water absorptive composite material by:
(a) applying an aqueous solution of a polymerizable monomer comprising as a main component acrylic acid, of which 20% or more of the carboxyl groups have been neutralized to its alkali metal salt or ammonium salt, to a prefabricated fibrous substrate, PA1 (b) polymerizing the polymerizable monomers applied to said fibrous substrate by means of a water-soluble radical polymerization initiator to form a composite of a polymer derived from said polymerizable monomer and said fibrous substrate, and PA1 (c) irradiating said composite with electromagnetic radiation or corpuscular ionizing radiation to obtain a water absorptive composite of which the water absorption velocity is enhanced as compared with the composite from step(b).
This process requires two distinct polymerization steps, and further requires that the second, high energy level polymerization result in an enhanced absorption velocity of the final product relative to the composite resulting from the first polymerization step.
Consequently, there remains a long felt and unfulfilled need in the cable industry for an effective yet inexpensive water blocking composition that is easy to handle and facilitates fast line speeds. A further need is for a water blocking composition that is not bulky or heavy so as to help maintain cable size and weight at a minimum. A desirable water blocking composition also would not pose the potential problems associated with delamination or loss of water absorbing material upon application to the cable. A water absorbing composition and method is also needed where the water absorbing material add on weight can be accurately controlled and will be maintained in the composition through subsequent processing and handling.